Effect of channel size on turbulent flow in tight lattice

The turbulent flow in 37 rod bundles is investigated with CFD code FLUENT. The calculation results are validated with experimental data at first. Then the effects of diminishing rod-rod gap size on turbulent mixing and coherent structure in tight lattice are analyzed. As the P/D decreases, the large scale coherent structure becomes more significant, and the resulted transverse momentum and energy exchange become more obvious. When the critical pitch to diameter ratio is about 1.03, the heat transfer in this kind of tight lattice is at the best. As P/D=1.01, there is nearly no large scale coherent structure in the tight lattice. There is also nearly no momentum and energy exchange and the heat transfer is not very good. (authors)